U.S. Pat. No. 5,750,585 (Park et al), the disclosure of which is incorporated herein by reference, describes certain superabsorbent hydrogel foams comprising a solid phase and a gas phase, in which the volume of the gas phase exceeds the volume of the solid phase. Such foams may generally be thought of as relatively light foams. The preferred density of the foams is stated to be between 0.015 and 0.5. Higher densities are stated to be undesirable as the swelling of the foam is slower (prior art, column 7, lines 35 to 46).
The prior art foams are stated to have potential utility as superabsorbents, oral drug delivery vehicles and gastric retention devices for diet control.
Hydrogel foams of polyacrylamide, polyvinylpyrrolidone, poly-(2-hydroxyethyl-methacrylate) or poly-(2-hydroxypropyl-methacrylate) are specifically mentioned.
The particular foams described in the said prior art document do not contain any organic plasticiser and are dried to provide superabsorbency. They are generally formed by polymerising at least one suitable hydrophilic olefin monomer compound in an aqueous solution containing a surfactant and about 0.1 to about 10% by weight of a crosslinking agent having at least two alkenyl groups; introducing gas into the monomer solution during the polymerisation step to form the foamed polymer matrix; and drying the foam.
The Examples of the said prior art patent show the use of sodium bicarbonate as a carbon dioxide blowing agent to generate the gas, although the general description mentions also mechanical introduction of gas into the monomer solution. The introduction of gas into the monomer solution during the polymerisation step is inconvenient, and would generally limit the polymerisation procedure to small batchwise production.
The foams described in U.S. Pat. No. 5,750,585 swell slowly on contact with water, typically over a time period of about 1 to 3 hours (see the Figures in the prior art patent). This slowness of water uptake makes the foams unsuitable for use in the applications contemplated in the present invention. The relatively low density of the foam makes it unsuitable for forming into films and sheets having acceptable mechanical strength.
U.S. Pat. No. 6,136,873 (Hahnle et al), the disclosure of which is incorporated herein by reference, describes certain superabsorbent hydrogel foams. The preferred density of the foam is stated generally to be between 0.05 and 0.7 g/cm3.
The prior art foams are stated to have potential utility as superabsorbents in diapers, sanitary towels and incontinence articles, and in certain other conventional uses for superabsorbents. Dressing material for covering wounds is mentioned as one potential application (column 15, lines 24 to 26).
The prior art document contains extensive lists of possible monomers and monomer mixtures for use in the polymerisable mixture. However, all the examples use a mixture of acrylic acid and sodium acrylate.
The particular foams described in the said prior art document may contain certain plasticisers and are stated to be usually dried after polymerisation, preferably to a water content of between 15 to 35% by weight.
The gas introduced into the monomer mixture is stated to be “fine bubbles of a gas inert to free radicals”. Examples show the use of mechanical stirring under an atmosphere of argon or carbon dioxide.
The foams described in U.S. Pat. No. 6,136,873 swell on contact with water, the absorption speed being reported as the parameter AS in the Examples. As used therein, AS=20/t, where t=the time for a 1 g piece of the foam to absorb 20 g of water (i.e. a 2000% uptake). While the water uptake rate appears to be faster than the foams reported in U.S. Pat. No. 5,750,585, the manufacturing process is inconvenient in view of the need for an inert gas atmosphere, and is most suitable only for batchwise production.
A large amount of research has been conducted into unfoamed, relatively non-porous, hydrogels based on hydrophilic polymers, e.g. for use as skin adhesives for a range of applications in skin-adhesive articles. Such materials exhibit a range of properties which make them suitable for skin adhesives. Representative references include PCT Patent Applications Nos. WO-97/24149, WO-97/34947, WO-00/06214, WO-00/06215, WO-00/07638, WO-00/46319, WO-00/65143 and WO-01/96422, the disclosures of which are incorporated herein by reference.